Hi there, just incase this helps, some implants make pregnancy tests and STI/STD tests show up negative when they are actually positive.
I would go talk to your GP about it properly becuase even if yoru not it could be something else like a cyst that is causing your stomach to swell up. I had a cyst and it made me go from a size 10 to a large size 12 and made me need to pee allot so i was worried too haha.
It would be a load of your mind to find out either way. It could even be that your soooo stressed that you may be pregnant that your subconciously thinking up more sympotons as your hyper sensitive to that fact that you could be.
I would strongly suggest seeing a GP or popping into a sexual health clinic just so you can be sure =) Hope you get whatever it is sorted out
Mx